There is a rapid, increasing need to manufacture biodegradable polymers to address the global dependence on non-biodegradable, petroleum-derived products.1 Current reasons for the popularity of petroleum-based non-biodegradables include superior modulus and yield strength in conjunction with high-impact strength.2 However, petroleum-based products, such as polystyrene and polyethylene, are limited in their commercial and industrial use, due to their high combustibility and non-biodegradable properties, which significantly increase the volume of landfills.3 In addition, recent standards set by the Underwriters Laboratories have specified the degree of flame retardancy for industrial materials such as polystyrene and polyethylene as UL94.4-5 Therefore, it is imperative to create a biodegradable polymer that is strong, flame retardant and has superior modulus.
Polylactic acid (PLA) and Ecoflex®
Biodegradable polymers exhibit many of the same mechanical and structural properties as petroleum-based products. Replacing petroleum-based products with bioplastics is projected to reduce air pollution by minimizing the need for synthetic polymer production in a cost-effective fashion environmentally sustainable.7-10 Using more biodegradable materials in industrial applications has been estimated to successfully reduce landfill mass by as much as 30%, plastic wastes by as much as 3,550 tons per year, and fossil fuels by as much as 4,010 tons per year.11,12
As biodegradable plastics have gained increased appeal, concern has risen regarding their flame retardant properties. Polylactic acid (PLA) and Ecoflex® are two biodegradable polymers which have become more popular in recent years due to their mechanical properties.13 However, the challenge is to control their mechanical properties in order to tailor them to different applications. One of this study’s goals was to create flame retardant biodegradable polymers which meet current industrial standards while minimizing loss of mechanical properties. These are seemingly contradictory conditions, since flame retardant formulations are known to embrittle materials.14 Here, exploration of the interactions of clays with PLA and Ecoflex® polymers to determine the optimal composite of mechanical strength, ductility, and flame retardance was examined.
PLA, a semicrystalline polymer (Figure 1), can be completely biodegradable in nature, thereby rendering it suitable for environmental applications, as a support for products which cannot revert to their original forms.15-17 PLA is also very heat-resistant, as evidenced by its use as a major component of thermoplastic trays.18 It is effective in solving the problem of plastic waste management posed by the traditional petroleum-based method, and has successfully produced 43% fewer greenhouse gases while using 48% less non-renewable energy than traditional plastic polymers.13
Similarly, Ecoflex®, another biodegradable material with a poly(butylene) adipate/terephthalate chemical structure (Figure 2), is made from oil-based compounds that function similarly to polyethylene (PE, Figure 3).16,19,20 PE, as the most commonly used plastic in the world, is found in various appliances, ranging from low voltage insulation cables, pipes, petrol tanks, car bodies, airplane parts and fuel tanks, plastic sheeting, carpet fibers, toys, and clothing.21-24 With this great use in the housing, clothing, and transportation industries, it has become imperative to design a new plastic with improved flame retardance as a replacement for commonly used petroleum-based products. This plastic would also have to be more environmentally-friendly, as High Density Polyethylene (HDPE) alone takes up approximately 6.3 million yds3 of landfill volume.25
Clay has been shown to be a naturally abundant, economical mineral that is toxin-free and possesses innate flame retardant properties.28-30 Clay oligomers offer a novel approach to improving the flame retardant (FR) properties of polymers via shear-induced exfoliation that circumnavigates toxicity issues.31 Materials such as resorcinol bis(diphenyl phosphate) (RDP) clay have also been used to reinforce biodegradable polymers.31-37 RDP assists flame retardance as an additive to bioplastics.31 To exfoliate clays with PLA and Ecoflex®, RDP would be ideal due to its chemical structure (Figure 4). It has surfactant properties with both nonpolar phenol groups and polar phosphoric acid groups, which are optimal for flame retardant formulations. Their phosphoryl groups act as strong hydrogen bond acceptors, and their phosphorus groups can react with polymer residue at very high temperatures.31
As a possible replacement for PE, PLA and Ecoflex® would be useful in multiple applications, due to their great durability, mechanical strength, and biodegradability. We propose a method of producing a flame retardant PLA and/or Ecoflex® material using nano-composite technology to answer the need for safe and costeffective biodegradable polymers.3,38
We propose a formula of producing a flame retardant PLA and/or Ecoflex® material using nanocomposite technology, incorporating RDP clay, instead of Cloisite clay, as an FR additive for testing of tensile strength, impact, modulus of elasticity, and flame retardance.
Materials and Methodology
Creating Thin Film Samples of PLA and Ecoflex® Nanocomposites
Polymer and additive interaction is important when optimizing the mechanical and thermal properties of the nanocomposite. Bulk think films were made to determine if the RDP clay interacted favorably with PLA or Ecoflex®.
To create samples of thin films that had both polymer substrate and RDP clay, the graduate student applied a monolayer of the clay onto the surface of a silicon wafer using the Langmuir-Blodgett (LB) Trough by increasing the surface tension of water on which clay particles were deposited. As the wafer was lifted out of the trough, the surface tension of the water pushed the clay layer onto the surface of the wafer.
PLA and Ecoflex® (both dissolved in chloroform) were used to create a spin curve to analyze the thicknesses of the substrates deposited on the silicon wafer. Ellipsometry was performed on silicon wafers coated with concentrations of 3 mg/mL, 5 mg/ mL, 10 mg/mL, 15 mg/mL, 20 mg/mL, 25 mg/mL, and 30 mg/ mL solutions of PLA and Ecoflex® to measure the thickness of the layers of substrate. The solution that produced the optimum thickness (approximately 1000 Å) was then used to spincast on wafers coated with a monolayer of RDP clay.
Surface Analysis of Thin Films through Atomic Force Microscopy
Silicon wafers were prepared by the graduate student to be cleaned twice with hydrofluoric acid to ensure that the wafer would prevent the water from spinning out with the excess PLA solution, and cleaned once for the hydrophobic Ecoflex®. These wafers were then coated with either 100% polymer (PLA or Ecoflex®) or polymer and RDP clay combinations of 10 mg/ mL (which equated to a measured length of approximately 1000 Å). The wafers in the latter category were then separately annealed in a vacuum oven for 18 hours at 170 °C to prepare samples for comparison. Atomic force microscopy (AFM) was then performed (with assistance from the undergraduate student) on these polymer surfaces, which allowed for observation of phase separation due to clay-polymer interaction
Further analysis of this polymer-clay combination was crucial to understanding the extent of exfoliation between the PLA/ Ecoflex® and the RDP clay. This would assist in determining the extent to which RDP clay was able to improve the mechanical structure and the consequent flame retardance. The surface tensions of RDP and the polymer in question (PLA or Ecoflex®) were used to yield the interfacial tension:31
Creating Bulk Samples of PLA and Ecoflex® Nanocomposites
Homogenous blending of PLA and Ecoflex® with resorcinol bis(diphenyl phosphate) clay was done using a C.W. Brabender instrument, type EPL-V501, with a direct current drive (type GP100).14 Initial blending of the bioplastic materials was started at 20 rpm at 170°C and gradually amplified to 100 rpm for 10 minutes after RDP clay was added to the chamber at the 2-minute mark. To bypass the issue of heat-activated polymer degradation, the homogenized blend was allowed a cooling period under nitrogen gas flow.
For this study, the copolymers were variegated into four main concentrations. The control (template) sample was the pure sample (0 wt% clay) of PLA and for Ecoflex®. The other treatments were set at gradual intervals of 1 wt%, 5 wt%, and 10 wt% RDP clay. Once polymers were extracted from the Brabender, they were shaped into molds and re-melted using a Carver heat press into flame-, tensile-, and DMA-appropriate samples. Both PLA and Ecoflex® samples were then removed.
UL-94 V0 Flame Retardancy Test
Flame tests determined the resistance of polymer compositions. Samples of 125 mm x 13 mm x 1.5 mm were created. A vertical burning chamber was set up with a stand and clamp. Rectangular planes used as flame test molds were vertically aligned so that their longitudinal axes were parallel to the stand, and samples were clamped at the upper 10 mm region of the stand. The lower end of the sample was above a 50 mm x 50 mm layer of 100% cotton, approximately 0.08 g in weight. An ASTM D5025 compressed methane gas burner, fueled by gas flowing at a rate of 105 mL/min, was used to generate a 20 mm blue flame. The flame was applied at an angle of 45° to the midpoint of the bottom edge of the flame test sample for 10 seconds. The flame was withdrawn immediately to more than 150 mm away from the sample while the afterflame time (t1) was recorded. Next, the flame was reapplied for another 10 seconds, after which it was removed while the second afterflame time (t2) was determined. The layer of cotton under the sample was then examined for any flaming particles remaining at the base.
Once this procedure was completed, the criteria presented in Table 1 were used to determine the classification of the material’s flame retardance level. In such a test, V-2 characterizes a flame retardant compound, V-1 is slightly more flame retardant, and V-0 is extremely flame retardant.
Characterization of Bulk Nanocomposites
An Instron 5566 tensile machine, set to an ambient temperature of 22 °C, determined the tensile strength of the polymer. First, a tensile sample was loaded onto the machine. A constant extension rate of 2 mm/min was used for the PLA composites while Ecoflex® samples were allotted a constant testing rate of 50 mm/min due to their higher elasticities. Once the flexural points had been determined, we obtained a complete tensile profile with a curve that indicated how the material reacted to applied forces.
In the initial portion of the test for most polymers, a linear relationship between the load and elongation was observed, as expected for a Young’s modulus. To determine the durability of the polymer-clay interactions and calculate the modulus of elasticity, we also measured the impact strength, i.e., the specimen’s potential energy.
Results and Discussion
To measure the interfacial interaction between clays and polymers, we first had to produce a polymer-clay bilayer to observe whether the polymer wet the clay surface (Figure 5).
Atomic Force Microscopy of Thin Films
The goal of this study was to find an efficient polymer-RDP interaction that would be flame retardant while maintaining the standard of mechanical efficiency of a petroleum-based product. Thus, Young’s contact angle was calculated to minimize energy and degree of exfoliation.
Scans of PLA and PLA/clay thin films, as seen in Figures 6 and 7, respectively, both reveal crystalline growth on the surface of the material caused by multiple nucleating sites. The centers of these points form spherulites as the crystals continue to grow independently until they become confined by other crystals and stop growing. This explains the extreme rigidity and hardness of PLA composites.
However, it is important to note that these spherulites show similar growth in both PLA and PLA/clay thin films, indicating that the surface of the PLA “wets” when coming in contact with the RDP clay monolayer.31 When annealing, the polymer approaches a state of minimum energy by either spreading flatly across the RDP-coated monolayer surface (wetting) or by forming droplets on the uneven interface (dewetting). Since the two PLA scans (with and without RDP) are similar, we can conclude that PLA spreads evenly onto the clay surface during annealing. This degree of wetting indicates that PLA and RDP clay are likely to exfoliate to near-completion when blended together.31
AFM scans of Ecoflex® thin films after annealing revealed the interactions between RDP clay and Ecoflex® and the composite material difference between the RDP/ Ecoflex® combination and pure Ecoflex®. As demonstrated in Figure 8, RDP nucleates crystals onto the surface of Ecoflex®, appearing as a rougher plane than the control seen in Figure 9. This leads to an observed increase in surface tension of the polymers as the properties of the material change, leading to crystallization into a hard polymer.
The increased roughness of the Ecoflex®/clay thin film can also be explained by a dewetting effect that annealing produces on the polymer.31 With Ecoflex®/clay films, a contact angle of 4.668° can be seen from a section analysis (Figure 10). This signifies that a small level of dewetting occurs, meaning that Ecoflex® may or may not exfoliate when combined with clay.31
We quantified these findings by examining the interfacial tension. At values approaching 0, the clay-polymer interaction is almost completely exfoliated. For sufficiently large values of tension, the extent of exfoliation varies, and the polymer may or may not exfoliate with the RDP monolayer. In the case of PLA, the surface tension is 50 mN/m. For Ecoflex®, we used the surface tension of PE (which closely resembled Ecoflex® in molecular structure), 31 mN/m. The surface tension of RDP at room temperature was calculated to be 49.9 mN/m
Therefore, the interfacial tension between PLA and RDP was calculated to be 0.1 mN/m. This indicates nearly uniform dispersal, indicating that “wetting” occurs, which in turn yields a negligible contact angle.31 However, for Ecoflex®, there is an interfacial tension of 18.9 mN/m. This value is significantly greater than 0, and so dewetting occurs. The level of exfoliation between Ecoflex® and RDP will be examined in greater detail later.
Determining flame resistant properties through UL flame tests
Flame tests of 0-10 wt% RDP clay and polymer composites revealed an improvement in flame retardance with the addition of 1-5 wt% clay (Table 2). Flame tests of PLA/clay nanocomposites showed a decrease in t1 values and an increase in t2 values with the addition of as little as 1 wt% RDP clay to the polymer base. However, the average t2 time remained constant thereafter as the percent composition of clay increased to 5 wt%. Flame retardant properties decreased with the addition of more clay after 5 wt%, as can be seen with the increase in t2 drip time in the 10 wt% results. For PLA/clay composites, the greatest flame retardant ability is observed with 5 wt% RDP clay since there is significantly reduced dripping. This allows for close to V-0 flame rating for the composite.
Ecoflex® composites revealed great improvement in t1 times with the addition of as little as 1 wt% RDP clay, bringing about significantly reduced dripping. However, this improvement did not continue as clay concentration increased from 1 wt% to 10 wt%. It is also significant to note that while t1 times for Ecoflex® composites showed rapid improvement, this came at the cost of t2 afterflame times. This trend was then reversed with the raise to 5 wt% clay Ecoflex® samples. At a concentration of 5 wt% clay, the total afterflame time reached a minimum and started to increase as more clay was added to the composites. Despite having relatively low afterflame times at 5 wt% clay, dripping was still observed, signifying that the flame rating remained at V-2.
The optimal concentration by weight of RDP to PLA is the same as the optimal concentration for Ecoflex® and RDP, both at 5 wt% composite. For both polymers, the average t1 and t2 times were lower than those from pure, 1 wt% clay, and 10 wt% clay samples. In fact, although Ecoflex® exhibited low variation in flame retardance, PLA showed a drastic improvement at 5 wt% RDP, reaching V-0. This is a significant finding since petroleum-based non-biodegradables are not usually flame resistant, and elevating them to the highest standard of UL94 is a major accomplishment. These findings support our hypothesis of RDP being an effective flame retardant additive for biodegradable polymers.
Mechanical characterization of polymer composites
To determine the mechanical characteristics of bulk samples of PLA and Ecoflex®, we compared tensile and DMA data to flame test data to determine the degree of improved flame retardance. This allowed us to identify whether the increased flame retardance needed to achieve an improved UL flame rating resulted in a decrease in mechanical efficiency.
PLA/clay nanocomposites showed a sharp decrease of 17.7 % in tensile strength with the initial addition of clay (Figure 11). These decreases in mechanical characteristics continue as the wt% of RDP clay increases but at a slower rate. A brief summary of the tensile properties is provided in Table 3.
A steady increase in the modulus of elasticity up to 5 wt% clay can also be observed from the initial slopes of the tensile stress-strain curve. Although this modulus fluctuation is minimal, the greatest elastic properties are present at a 5 wt% concentration, coinciding with the lowest impact strength (Figure 12). This reinforces the idea that PLA exfoliates well in RDP as the incorporation of clay embrittles the composite, observed by the decrease in impact strength, and prevents dripping, observed with reduced afterflame times and attainment of the V-0 level at 5 wt%.
However, beyond 5 wt% RDP clay, Young’s modulus showed a drastic drop of 8.88% (Figure 13). Addition of as little as 1 wt% RDP clay leads to a large decrease in initial mechanical properties in PLA/clay composites. In fact, the impact strength of the PLA/ clay composites decreases by a factor of 21.3% upon initial administration of RDP clay.
Stress-strain curves for the Ecoflex®/clay films with different concentrations of RDP clay are shown in Figure 14. The corresponding tensile properties are summarized in Table 4. The addition of clay particles has a profoundly inverse effect on the tensile properties of the final material. The impact strength continues to decrease without reaching a lower boundary (Figure 15) although the modulus in this case is also relatively the same (Figure 16).
This is also supported by the flame test results as there was no change in dripping for the Ecoflex®/clay composites among different concentrations. When considered alongside the interfacial tension results, Ecoflex® appears to be largely incompatible with RDP clay additives.
For the Ecoflex®/clay composites containing 1 wt% clay, the tensile strength at break is 5.3% lower than that of pure Ecoflex®, and it does not considerably decrease in the transition from 0-5 wt% clay. However, with the addition of 10 wt% clay, tensile strength drops by 20% compared to that of 5 wt% samples. In addition, mechanical variation of the polymer can be observed with the 10 wt% samples in the tensile. Finally, the impact strength for Ecoflex® had an initial drop of 39.1% upon RDP clay administration.
The bilayer made between the polymer and the clay produced by the LB-technique not only effectively predicts whether the clay will exfoliate in a polymer matrix, but does so in a cost-effective manner by requiring small amounts of polymer.
Conclusion and Future Investigation
We have demonstrated that surface energies are important in determining the overall mechanical properties of the nanocomposite. Through careful matching of the surface energies, it is possible to use clay to engineer new materials with optimal mechanical and thermal properties. For example, in the case of PLA/clay, PLA wets the surface of the RDP clay, so RDP clay exfoliates in PLA. This was confirmed by the significant dependence of the impact of the modulus on clay concentration and the prevention of dripping during the flame test. Thus, flame retardance improved through lack of dripping at a cost to the impact strength.
In contrast, in the case of Ecoflex®, where the energy is unfavorable, we found little enhancement and continued dripping during the flame tests, yet an observed benefit was the constancy of the impact strength. Dewetting was observed as there was little or no effect on the impact strength, flame testing, and Young’s modulus.
UL94 V-0 flame tests indicated that the RDP clay’s effect on flame retardance of the composites peaked at 5 wt% clay in polymer. Although afterflame times decreased, the nanocomposites were only able to reach a flame level of V-2. It is likely that the addition of RDP clay nanotubes to the composites, forming a net-like structure on the atomic level, would significantly reduce dripping and allow the polymers to reach V-0.
AFM images of PLA/clay and Ecoflex®/clay films involved spinning a thin film of polymer on a monolayer of clay in order to study the interfacial tension. PLA was observed to completely wet clay, whereas Ecoflex® dewets with a small contact angle of 4.668°, indicating a less favorable interaction between the surfaces. Therefore, it is expected that the RDP clays will exfoliate easily in the PLA but not so in Ecoflex®. Cross-sectional scans of the composites using transmission electron microscopy should be performed to examine clay exfoliation. It would therefore be interesting to examine the effects of making clay tubes more flame retardant.
Tensile tests also confirmed a higher modulus of elasticity of the composites with addition of clay while showing a decrease in impact strength. Optimization of these properties occurred at 5 wt% clay due to a large increase in modulus while impact strength remained relatively stable. Tensile properties may be further improved by combining PLA with Ecoflex® and RDP clay. Whereas PLA combined with RDP enhanced flame retardance, the Ecoflex®/ clay composites did not show successful exfoliation with RDP. To improve the flame retardance for both, a flame retardant formulation based on RDP and aluminum trihydrate will be tested. These nanocomposites can then be engineered to resemble the toughness and elasticity of petroleum-based polymers such as polystyrene and PE.
Further uses of this investigation include a replacement for PE with biodegradable materials for use in construction, structure, clothing, and transportation. An additional goal would be to enhance both flame retardance and mechanical strength. To do this, we will meld PLA and Ecoflex® samples with RDP clay as an alternative for non-biodegradable petroleum-based products since PLA is a harder substance and Ecoflex® is softer. However, for both polymers, the ignition time is high, and additional tests could resolve flame retardant properties.
Finally, a current application that may be viable from this research is in Army Combat Uniforms.39 With the addition of natural, biodegradable plastics and FR additives like PLA/Ecoflex® and RDP clay, these suits may be made more flame retardant and durable. In this way, improvement upon biodegradable polymers, such as PLA and Ecoflex®, has thus far successfully demonstrated promising advances in serving as an alternative for nonbiodegradable petroleum-based products.
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